Lower .alpha.-olefin dimers, such as dimers of ethylene propylene, butene, etc., are usually used as a key material for the preparation of agricultural chemicals, perfumes, formed products, etc. and also as a starting material for producing high molecular weight compounds. It is known that the dimers can be produced by dimerization of monomer(s) by using the following nickel-containing Ziegler type catalyst:
(i) a catalyst comprising a .pi.-allyl nickel complex/an organic aluminum halide compound/an organic phosphine (cf. Japanese Patent Second Publication (Kokoku) No. 34007/1971),
(ii) a catalyst comprising a nickel salt/an organic aluminum halide compound/ an organic phosphine (cf. Japanese Patent Second Publication (Kokoku) No. 22807/1972), and
(iii) a catalyst comprising a nickel salt/a trialkylaluminum/an organic phosphine/a halogenated phenol/water (cf. Japanese Patent First Publication (Kokai) No. 167932/1982).
However, in the above process (i), there is used as a nickel component a .pi.-allyl nickel complex which is unstable to air, and hence, it is very difficult to handle and further, the complex must be prepared separately by a process having complicated steps.
Moreover, in either process (i) or (ii), an organic aluminum halide compound must be used as the aluminum component, and hence, the halogen atom bound to aluminum atom is easily removed therefrom in the form of HCl etc. due to moisture etc. contained in the system, which unfavorably induces corrosion of apparatus. When a lower olefin such as propylene is used, the fraction of 2,3-dimethylbutenes (hereinafter referred to as "DMBS") in the produced dimer is unfavorably low.
The process (iii) had been accomplished by the present inventors to overcome the above problems associated with the known processes. However, even by this process, there is unfavorably induced lowering of heat-removal capacity of the heat exchanger for cooling during the continuous operation for a long period of time. On analysis of this unfavorable phenomenon, it has been found that it is induced by deposition of fine precipitates on the surface of the heat exchanger which originate from the catalyst, and that the catalyst used in the process (iii) is not necessarily satisfactory in view of non-uniformity of the catalyst.
Thus, the known processes have various problems and are not satisfactory for the industrial production of dimers of .alpha.-olefins.